(a) Field of the Invention
The present invention relates to a control signal unit for a liquid crystal display and a method for fabricating the same and, more particularly, to a control signal unit for a liquid crystal display operating in a stable manner without line opening.
(b) Description of the Related Art
Generally, a liquid crystal display (LCD) has two glass substrates with electrodes, and a liquid crystal sandwiched between the substrates. When voltage is applied to the electrodes, the liquid crystal molecules are rearranged, thereby controlling light transmission.
One of the substrates has color filters, and the other substrate has thin film transistors (TFTs). The former substrate is usually called the “color filter substrate,” and the latter substrate called the “TFT array substrate.”
The display area is positioned at the center of the TFT array substrate. In the display area, a plurality of gate lines are formed in the horizontal direction, and a plurality of data lines cross over the gate lines to form pixel regions in a matrix type. The TFT is formed at each-pixel region together with a pixel electrode such that it is electrically connected to the gate line and the data line. The TFT controls the data signals from the data line in accordance with the gate signals from the gate line, and sends the controlled signals to the pixel electrode.
A plurality of gate pads and data pads are formed externally to the display area such that they are connected, on the one hand, to the gate lines and the data lines, and on the other, directly to external driving ICs. The gate pad and the data pad receive the gate signal and the data signal respectively from the driving ICs, and send them to the gate line and the data line.
A gate printed circuit board, and a data printed circuit board are connected to the TFT array substrate to transmit the gate signal and the data signal thereto. Data signal transmission films interconnect the TFT array substrate and the data printed circuit board while mounting with data driving ICs for converting electrical signals into data signals and outputting the data signals to the data lines. Furthermore, gate signal transmission films interconnect the TFT array substrate and the gate printed circuit board while mounting with gate driving ICs for converting electrical signals into gate signals and outputting the gate signals to the gate lines.
Alternatively, without a gate printed circuit board, the data printed circuit board may output the gate control signals to the gate driving ICs of the gate signal transmission films via the TFT array substrate, thereby controlling the gate driving signals.
The gate control signals include various kinds of control signals such as gate on voltages (Von) and gate off voltages (Voff), and common voltages Vcom.
The control signal lines carrying such gate control signals are formed with a low resistance conductive material capable of rapidly carrying the signals. Aluminum is commonly used for that purpose, but bears unstable physical and chemical properties. Therefore, the control signal lines have a double or triple-layered structure with an aluminum-based layer and other layers based on metallic materials bearing relatively high resistance.
In case indium tin oxide (ITO) is used to form pixel electrodes and pads, since the aluminum-based material bears poor contact characteristic with respect to the ITO, the aluminum-based layer should be removed at the contact area.
The control signal lines may be processed in the following way. A metallic layer and an aluminum-based layer are sequentially deposited onto a substrate, and etched through photolithography to form a double-layered signal line. An insulating layer is then deposited onto the substrate such that it covers the double-layered signal lines. Contact holes are formed at the insulating layer, and the aluminum-based layer of the signal lines exposed through the contact holes are removed through etching. Subsidiary pads are formed on the exposed portions of the metallic layer. In the processing step where the exposed portion of the aluminum-based layer is completely removed, the non-exposed portion of the aluminum-based layer under the insulating layer is partially etched inside of the insulating layer while forming undercut regions.
Meanwhile, when strong static electricity is generated at the device, the static electricity is accumulated at the gate off voltage line and the common voltage line bearing relatively high capacity. In the process of discharging the static electricity, surge current accruing to the discharge of the static electricity is flown along the gate off voltage line and the common voltage line, and this generates Joule heat.
Particularly, the voltage drop is intensified at the undercut regions because an aluminum-based layer or other conductive layers capable of receiving the static electricity is absent at those regions. Accordingly, the voltage drop is focused at the undercut regions, and large amount of Joule heat is generated there. The Joule heat may melt the metallic layer and result in line opening.